1. Field of the Invention
The present invention is directed to radar systems in general, and in particular to such systems utilizing spread spectrum (SS) and pseudo-noise (PN), maximal length sequence technologies. More particularly still, it is directed to a radar system where compensation (partial cancellation) of the leaked transmit signal is accomplished at baseband of the PN code sequence. As such, the radar system is particularly useful in police radar gun, traffic monitoring and automotive collision avoidance applications, where the use of a single antenna for both transmit and receive is desirable.
2. Prior Art of the Invention
U.S. Pat. No. 5,657,021 (commonly owned by the present assignee) for a SYSTEM AND METHOD FOR RADAR VISION FOR VEHICLES IN TRAFFIC, issued Aug. 12, 1997 discloses interference-free radar systems utilizing PN waveforms for sequential transmission by radar, wherein the PN waveform is tapped and adjustably attenuated to cancel leakage within the system prior to correlation of the received echo.
A paper titled A COLLISION AVOIDANCE RADAR USING SIX-PORT PHASE/FREQUENCY DISCRIMINATOR (SPFD) by Ji LI et al published May 23, 1994 in 1994 IEEE MTT-S Digest, pp. 1553-1556, proposed a novel technique for collision avoidance radar used in automobiles, in which a new six-port microwave/millimeter wave digital phase/frequency discriminator (SPFD) is used to measure Doppler frequency shifts. Both relative speed and moving direction of the target are readily obtained. Ranging is implemented by the measurement of phase difference at two adjacent frequencies.
In this paper by LI et al state:
xe2x80x9cIn CW type radars, one of the most serious problems is to achieve sufficient isolation between transmission and reception. To prevent the receiver from saturation, separate transmitting and receiving antennas are often used. This results in unwanted larger volume and higher cost. Some other solutions such as Reflected Power Canceller (RPC) [6] are proposed and implemented, however the cost and complexity are still high. In contrast, by using new SPFD it is very easy to integrate a RPC into the sensor at the expense of only a vector modulator (phase shifter and attenuator). The single antenna scheme is shown in FIG. 3. In the six-port PFD, the leakage of the transmitted signal yields a deviation of the detected vector from the origin. A feedback algorithm can be adopted to control the loop to realign the vector to the origin, such that the leakage power is canceled outxe2x80x9d.
In UK patent application GB 2,268,350, published May 1, 1994, for HIGH RANGE RESOLUTION RADAR a phase-coded signal is transmitted by one antenna and the reflections received by another. Both the outward and return signals are mixed in a quadrature mixer to produce a baseband replica of the coded signals, which are then filtered and, amplified before being applied to a correlator. Internal signal leakage in this system does not appear to be a problem.
In U.S. Pat. No. 5,134,411, issued Jul. 28, 1992, a NEAR RANGE OBSTACLE DETECTION AND RANGING AID apparatus is disclosed. Range measurement signals are produced by means of phase comparison of signals in two paths. The subject of xe2x80x9cLeakage Correctionxe2x80x9d is discussed as follows:
xe2x80x9cIn a practical system one or more leakage paths may exists between the RF and LO ports of the mixer. When measuring a target with a weak echo signal, a stronger leakage signal may cause significant errors. Since the transformation 13 has a commutative property, we can generate a corrected signal ucorr(i)=u(i)xe2x88x92ucal(i), which is to be used in equation 13. The signal uca(i) is measured when no targets are present. Alternatively, we can measure uca(i) even in the presence of targets, if both antennas are replaced by a matched load. In this case, however, the external leakage between the antennas cannot be corrected and therefore will limit the useful dynamic range of the target echoxe2x80x9d.
The issue of leakage in the circulator in FIG. 7, where a single antenna is used, is not addressed.
In a paper by Yukiko HANADA et al titled VEHICULAR SPREAD SPECTRUM RADAR FOR MULTIPLE TARGETS DETECTION USING MULTI-BEAM ANTENNA (IEIC TRANS. FUNDAMENTALS, VOL. E-80, NO. 12 DECEMBER 1997), the author propose and investigate a vehicular radar system that can measure the distance to, the relative speed of and the direction of arrival (DOA) of the reflected waves from multiple targets or vehicles using the direct-sequence spread spectrum (DS-SS) technique. In particular, they propose a DOA estimation scheme using a multi-beam antenna. In order to show that the proposed system can accurately measure the above-mentioned quantities, the performance is evaluated numerically in a multi-path environment. Moreover, the optimal multi-beam pattern is derived to minimize error probability of DOA estimation. The author state that they use several antennas which form sharp multiple beams, which can be implemented by using several types of antennas such as phased array antenna and a combination of directional antennas.
In a paper titled 76 GHZ AUTOMOTIVE MILLIMETER-WAVE RADAR USING SPREAD SPECTRUM TECHNIQUE by Hiroshi ENDO et al, published in SAE TECHNICAL PAPER SERIES 1999-0102923, the author state:
xe2x80x9cIn SS radar, transmission signals are modulated using PN codes, and then transmitted through the transmission antenna. The signal reflected from a target located ahead of the radar equipped vehicle has a time delay that corresponds with the two-way range delay, the Doppler shift corresponds with the range rate between the radar equipped vehicle and a target ahead; and that signal that is received by the reception antenna. The PN sequences have an auto-correlation function as shown in FIG. 1 [2]. Utilizing these characteristics, SS radar can measure range from the phase difference of PN sequences. The range rate can be measured by frequency analysis when the correlation peak is detected. In this method, accurate raging and multiple target separation are possible due to the detection method using the auto-correlation characteristics of PN sequences. Moreover, SS modulation has excellent interference capabilities since the demodulation process using PN sequence spreads undesired signals or interference in the channel and thus suppresses those signalsxe2x80x9d.
Finally, in a paper titled SYSTEM ASPETS AND DESIGN OF AN AUTOMOTIVE COLLISION WARNING PN CODE RADAR USING WAVEFRONT RECONSTRUCTION By Jxc3xcrgen DETLEFSON et al, published in 1992 IEEE MTT-S Digest, pp.625-628, the author disclose a 61 GHZ radar system with the following parameters:
The radar systems of the present invention have some of the features of prior art systems. In a preferred implementation of the present invention, the radar is based on a CW carrier phase modulated with a maximal length PN code sequence providing a low power spread spectrum signal. Target range is determined by correlating the radar return signal with a delayed copy of the transmitted PN code.
An important feature of the present invention is signal leakage compensation by means of signal feedthrough cancellation techniques. Such compensation, while generally useful, is particularly desirable for compact radar systems, whether for law enforcement applications (police radar gun) or for automotive and similar applications. However, while the system of the present invention is particularly suitable for single antenna radars, it is still applicable where separate transmit and receive antennas are used. In such applications, it would permit improved performance, for example, range increase and/or decreased receiver dynamic range requirements.
A brief general description of the theory of operation of the spread spectrum system provides a better understanding of the basis for the unique features of the system. The following features are directly derived from the exploitation of PN maximal sequences as the class of modulation waveforms that are used in the present radar. These features are:
Direct, simple and accurate range measurements to a target;
Immunity to electromagnetic interference;
Range resolution is easily changed;
Lower probability of detection;
Immunity from mutual interference in the presence of other spread spectrum radars which are in close proximity; and
Implementation is independent of the frequency band of the carrier signal.
The following properties of PN maximal length sequences contribute to these features.
A PN maximal length sequence has an autocorrelation function which is a periodic triangle of height equal to N and a base 2/fowide, and a level of xe2x88x921 between the triangles, where fo is the code clock frequency and N is the length of the code. The width of the auto correlation function as defined by the clock frequency enables accurate range measurements. Maximal PN sequences may be generated from an n-stage shift register with feedback points. The register length n determines the number of different maximal PN codes generated as per Table 3.3 on page 72 of SPREAD SPECTRUM SYSTEMS, Third Edition, by Robert C. Dixon (John Wiley and Sons, Inc.); this book is incorporated herein by reference.
External interference that does not auto-correlate with the code has its energy spread out over a large bandwidth by being a cross-correlation with the code, the result is that interference is attenuated by a factor of xe2x80x9cNxe2x80x9d. This is an important consideration because of the current proliferation of mobile communications emissions that can interfere with a radar""s operation.
For each length of code there is a large number of codes available to be used and each has the same optimal autocorrelation function property. Since the cross-correlation between the codes is very low and of the order of 1/N, therefore, in a situation where there are a number of radars working in close proximity of each other, if each radar uses a different code, then there is minimal interference between these radars. The selection of codes can be implemented in a very convenient manner. This property is very important when considering applications such as automobile collision avoidance, traffic detection/management, perimeter surveillance, etc where a multiplicity of radars are used in close quarters.
In a spread spectrum radar the transmitter output power is spread over a large bandwidth determined by the code generator clock frequency. The net result is that the power spectral density is considerably reduced (approximately by a factor of 1/N) to such an extent that current state-of-the-art radar detectors cannot detect such low powers per unit frequency. This is an important consideration for police radar guns.
Preferably, the spread spectrum radar measures both the Doppler velocity and range to a target. The measured Doppler velocity is a function of target speed and the angle between the radar and the track of the target. Normally, the angle is limited less than 10 degrees to minimize the measurement error. Beyond 10 degrees the error becomes unacceptable. By measuring the Doppler velocity and range of a target at two points on the target""s track, the target""s velocity can be computed accurately up to an angle of at least 40 degrees. This is particularly relevant to the police radar gun application.